1. Field of the Invention
This invention relates to a pre-filtering apparatus for effecting a preliminary filtering for use in conjunction with a continuous press.
2. Description of the Related Art
Such an apparatus is used to filter water, oils and fats out of raw materials to a certain extent prior to the filtering operation by means of the continuous press. By effecting the pre-filtering, pre-pressed materials having a substantially constant liquid concentration can be introduced into the continuous press so that the high filtering efficiency can be attained.
In order to continuously press water, oils and fats out of raw materials such as paper pulp, bony parts, guts and sludges, there has been widely used a continuous press of a screw press type. In general, the continuous screw press comprises a screw shaft arranged rotatably, a screw drum arranged around said screw shaft to rotate therewith, a screw blade arranged helically around the outer surface of said screw drum, an outer drum arranged around said screw blade and having a number of small filtering holes formed therein, an inlet provided at one end of said outer drum and an outlet provided at the other end of said outer drum, whereby raw materials to be pressed are introduced through said inlet into a space between said screw drum, screw blade and outer drum, said space being gradually made smaller in a direction from said inlet to said outlet.
In case of effecting the filtering operation with the aid of such a screw press, when the raw material has a substantially constant liquid concentration, the filtering can be carried out efficiently. However, when the water concentration of the raw material is varied during the operation, the filtering efficiency might be decreased. Therefore, when the raw material has a varying liquid concentration, it is preferable to effect a preliminary filtering for the raw material prior to the introduction of the raw material into the screw press so as to make the liquid concentration of the raw material as constant as possible. Heretofore, in order to suppress the variation of the liquid concentration of the raw material, there has been used a rotating sieve, a drum screen and the like separately from the screw press. However, such known pre-filtering apparatuses are large in size and a large space is required for installing the pre-filtering apparatus. Further, the cost of the installation and the running cost become extremely high. Moreover, the maintenance of the known pre-filtering apparatus requires a high cost and much labor work.